At present, there exists a multitude of electro-optic modulators (EOM), which are used to modulate the amplitude, frequency, phase and polarization of a beam of light. Among these, phase modulation provides the highest quality of transmitted signal, though at the expense of a widened spectrum. In view of the benefits, optical phase modulation has various applications in the field of optical networks and data transmission.
The most common phase modulator uses a Lithium Niobate crystal (LiNbO3), which has an index of refraction that depends linearly on the applied electric field, and a phase linearly dependent on the index of refraction. As the electric field changes, the resulting phase is modulated. The achievable variation of the refractive index in Lithium Niobate is relatively small, requiring either large voltages or long electrode lengths to obtain sufficient phase modulation. Such modulators may also perform the task of amplitude modulation.
ThorLabs™, for example, produces Lithium Niobate phase modulators made of Titanium Indiffused Z-Cut LiNbO3, which are especially designed to be integrated into transponders. The Lithium Niobate component is required for all-optical frequency shifting, and applications such as sensing and data encryption. These phase modulators are designed to operate in the 1550 nm range.
Jenoptik™ produces integrated optical phase modulators, which employ a combination of Magnezium oxide (MgO) and Lithium niobate (LiNbO3) crystals to realize phase modulation in the GHz range. An advancement with Jenoptik™ phase modulators is that a relatively low modulation voltage is required to achieve the desired phase modulation, thus being suitable for wavelengths in the visible and infrared spectral range.
Other methods of optical phase modulation have also been employed. For example, optical phase modulation has been achieved in a traveling wave semiconductor laser amplifier, see paper by Hui, R. Jiang, Q. Kavehrad, M. Makino, T., “All-optical phase modulation in a traveling wave semiconductor laser amplifier”, IEEE Photonics Technology Letters 1994, vol 6(7). In this paper, the optically controlled phase modulation is independent of the signal wavelength.
In spite of advances made in the area of optical phase modulation, there is still a need in the industry for developing further improvements and alternative methods of optical phase modulation and optical networks using the same, which would avoid or mitigate the disadvantages of the existing prior art.